Method of Manufacturing a Solenoidal Magnet, and a Solenoidal Magnet Structure

ABSTRACT

A method of manufacturing a solenoidal magnet structure, comprising the steps of providing a collapsible mold in which to wind coils; winding wire into defined positions ( 88 ) in the mold to form coils ( 34 ); placing a preformed tubular mechanical support structure ( 102, 120 ) over the coils ( 34 ) so wound; 10 impregnating the coils and bonding them to the mechanical support structure by applying a thermosetting resin and allowing the thermosetting resin to harden; and collapsing the mold and removing the resultant solenoidal magnet structure comprising the resin impregnated coils and the mechanical support structure from the mold as a single solid piece.

The present invention relates to a method of manufacture of solenoidalmagnet coils, and to solenoidal magnet coils themselves. In particular,it relates to such coils for generating high strength magnetic fields,which may be applied in systems such as nuclear magnetic resonance (NMR)or magnetic resonance imaging (MRI).

FIGS. 1A-1B illustrate cross-sectional and axial sectional views,respectively, of a conventional solenoidal magnet arrangement for anuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI)system. A number of coils 34 of superconducting wire are wound onto aformer 1. The resulting assembly is housed inside a cryogen vessel 2which is at least partly filled with a liquid cryogen 2 a at its boilingpoint. The coils are thereby held at a temperature below their criticaltemperature.

The former 1 is typically constructed of aluminum, which is machined toensure accurate dimensions of the former, in turn ensuring accurate sizeand position of the coils which are wound onto the former. Such accuracyis essential in ensuring the homogeneity and reliability of theresultant magnetic field. Superconducting magnets may quench due to evena small amount of movement of even one turn of the coil. The formersmust therefore be very rigid. These requirements combine to render theproduction of formers very expensive.

Also illustrated in FIGS. 1A-1B are an outer vacuum container 4 andthermal shields 3. As is well known, these serve to thermally isolatethe cryogen tank from the surrounding atmosphere. Insulation 5 may beplaced inside the space between the outer vacuum container and thethermal shield. However, as can be seen in FIGS. 1A-1B, these elementsalso reduce the available inside diameter 4 a of the solenoidal magnet.Since the available inside diameter 4 a of the solenoidal magnet isrequired to be of a certain minimum dimension to allow patient access,the presence of the outer vacuum container 4 and the thermal shields 3effectively increases the diameter of the magnet coils and the former 1,adding to the cost of the overall arrangement.

The cost of producing a former 1 such as illustrated in FIGS. 1A-1B anddescribed above is accounted for approximately equally by labor costsand material costs. Among other objectives, the present invention seeksto reduce the labor costs involved in producing a solenoidal magnetstructure.

U.S. Pat. No. 5,917,393 describes a superconducting magnet arrangementwherein superconducting wire is mounted on an inner or outer surface ofa thermally conductive cylinder, whereby cooling may be applied throughthe material of the former. The superconducting wire is thermallyconnected to, but electrically isolated from, the material of thecylinder.

The present invention aims to alleviate at least some of the problems ofthe prior art, and provides a relatively inexpensive and lightweightsolenoidal magnet structure which is capable of withstanding the forcesapplied to it in use, and provides accurate and stable positioning ofthe coils of the solenoidal magnet.

Accordingly, the present invention provides apparatus and methods asdefined in the appended claims.

The above, and further, objects, characteristics and advantages of thepresent invention will become more apparent from consideration of thefollowing description of certain embodiments thereof, given by way ofexamples only, in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1B show a solenoidal magnet structure housed within a cryostat,according to the prior art;

FIG. 2 shows an overall perspective view of an assembly of coils and amechanical supporting structure according to an embodiment of thepresent invention;

FIG. 2A shows an overall perspective view of an assembly of coils and amechanical supporting structure according to another embodiment of thepresent invention;

FIG. 3 shows an axial half-cross-section through an assembly of coilsand a mechanical supporting structure according to an embodiment of thepresent invention; and

FIGS. 4A-4D show steps in a method for producing an assembly of coilsand a mechanical supporting structure according to an embodiment of thepresent invention.

The present invention particularly provides a method of manufacturing asolenoidal magnet structure, An embodiment of the present invention willnow be discussed with reference to FIGS. 2-4D.

FIG. 2 shows an overall perspective view of an assembly of coils 34 anda mechanical supporting structure 102 according to an embodiment of thepresent invention. The invention provides that the mechanical supportingstructure 102 is a preformed cylinder, such as of a metal or a compositematerial, bonded to the radially outer surface of the coils. Typicalexample materials include aluminum, and fiberglass reinforced epoxyresin. Coils 34 are formed within a mold, the mechanical supportingstructure 102 is slid over the outside of the coils, and the wholestructure is then resin impregnated to bond the coils onto the supportstructure and produce a single solid piece which comprises the coils 34and the mechanical support structure 102.

FIG. 2 shows an example of a cylindrical mechanical support structure102 as used in the present invention. As shown, the structure 102 mayhave an axial split 103 along its length, bordered with retainingflanges 104. When assembling a magnet structure of the presentinvention, the axial split 103 may be opened, to increase the internaldiameter of the support structure 102. The support structure may then beslid easily over the coils 34. Once in position, the retaining flanges104 may be pulled together, for example using bolts, clamps, screws orany other suitable arrangement. This reduces the internal diameter ofthe support structure, and may cause the internal surface of the supportstructure to bear onto coils 34, retaining them in position.

In an alternative arrangement illustrated in FIG. 2A, the cylindricalmechanical support structure 102 is a complete cylinder. The internaldiameter of the mechanical support is somewhat larger than the outerdiameter of the coils 34. To install, the support structure 102 is slidover the cons 34, leaving a clearance between the outer surface of thecoils and the inner surface of the mechanical support. This clearance isthen filled with a filler, for example a granular filler such as sand orglass beads, before the impregnation step. During the impregnation step,resin fills the clearance, around the filler, impregnates the coilwindings and causes the coils to adhere to the inner surface of thecylindrical mechanical support.

In both examples, as shown in FIG. 2 and FIG. 2A, mechanical supportstructures such as shown at 101 may be provided on the external surfaceof the support structure, for use in mounting the magnet structure—forexample within a cryogen vessel or vacuum vessel.

FIG. 3 illustrates an axial half-cross-section through an assembly ofcoils and a mechanical supporting. structure according to an embodimentof the present invention. An advantage of the arrangement of the presentinvention is that the frictional interface between the radially innersurface of the coils and former is eliminated. This frictional interfacemay provoke quench in prior art arrangements, since movement of a coilin frictional contact with a former may cause sufficient localizedheating to bring about a quench. The radially inner surface of asuperconducting coil is the part most susceptible to quench, so it isparticularly useful to eliminate frictional heating in that region.Since there are no such frictional interfaces in the arrangement of thepresent invention, such risks are not present. The coils are separatedfrom the mechanical support structure by filer layers 40, which alsoserve as thermal diffusion barriers, as will be explained below. Thebody force, that is the axial electromagnetic force acting on the coils,is restrained by the shear bond strength between the coils 34 and themechanical support structure 120. The interfacial shear strengthrequired between the coils 34 and the mechanical support structure 102to maintain their position when subjected to the electromagnetic loadsis within the capability of existing technology of bonding methodsdescribed herein.

In some conventional arrangements, slip planes were provided. These areinterface surfaces along which the coils can move relatively easily overa former. Rather than trying to prevent movement, their aim is to makeany movement as frictionless as possible to reduce the frictional heatproduced by any such movement. In the present invention, the coils arefirmly attached to the mechanical support structure, and there is noneed to provide slip planes.

As illustrated in FIG. 3, a thermal diffusion barrier 40 may be providedbetween the coils and the mechanical support structure. This thermaldiffusion barrier 40 should be electrically insulating, magneticallyinert and thermally conductive. The thermal diffusion barrier may be alayer of wire wound over the superconducting coil. The thermal diffusionbarrier serves to spread heat, to prevent heat from the former reachingthe coil, or at least to spread any heat from the former over a largersurface of the coil, to prevent hot spots. The thermal diffusion barriermay also serve to bring the outer diameter of certain coils 34 to nearerthe inner diameter of the mechanical support structure. The thermaldiffusion barrier may, as described further below, include a compositematerial such as fiberglass reinforced epoxy resin.

In the arrangement of FIG. 3, the three axially inner coils have asmaller outer diameter than the axially outer coils. This is believed tobe typical of current superconducting magnets for MRI imaging systems.Evidently, the support structure 102 cannot be slid over the axiallyouter coils, as it has an internal diameter less than the externaldiameter of the axially outer coils. The support structure may be splitaxially, as shown in FIG. 2, the cut 103 opened until the support 102can pass over the radially outer coils, then be clamped back togetheronce aligned with the coils 34. One alternative method for constructingsuch an arrangement, according to the present invention, is illustratedin FIG. 3A. The magnet assembly is constructed in three parts. A first,central, mechanical support structure 102 a is assembled to the axiallyinner coils of reduced internal diameter. Second and third, outer,mechanical supports 102 b, 102 c are then assembled to respectiveaxially outer coils. The three parts of the mechanical support structuremay then be assembled together, and the whole assembly impregnated.Radial flanges 105 may be used to join the parts together. Flanges 105may be joined using bolts, clamps, screws or any other suitablearrangement. In an alternate method, illustrated in FIG. 3B, a first,smaller diameter, mechanical support structure 102 d is assembled to theaxially inner coils of reduced internal diameter. A second, largerdiameter, mechanical support structure 102 e is then slid over the firstmechanical support 102 d and the axially outer coils. The resultingassembly is then impregnated. It may be preferred that the second,larger diameter, mechanical support structure is axially split, as shownin FIG. 2. This will allow an increase in the internal diameter of themechanical support structure to allow it to slide easily over the first,smaller diameter, mechanical support structure. The split 103 may thenbe closed, as discussed with reference to FIG. 2.

Another alternative arrangement for forming the mechanical supportstructure is schematically illustrated in radial cross-section in FIG.3C. A plurality of sections 106, each of arcuate cross-section, areassembled together to create a cylindrical support structure. They maybe assembled using axial flanges 107 running along the edges of eachsection. As the flanges 104 of FIG. 2, the flanges 107 may be joinedtogether using bolts, clamps, screws or any other suitable arrangement.

Patient comfort and accessibility of clinicians in MRI systems may bothbe improved by reducing the length of the magnet. Coil arrangementswhich permit this length reduction, while maintaining field quality mayresult in coil body forces which act in an axial direction away from thecentre of the magnet. Conventional methods of restraining these forcesrequire additional former material to be positioned on the end of thecoil, which increases the length of the magnet system. By utilizing theshear strength at the interface between the coils and the mechanicalsupport structure, such additional material can be avoided and a shortermagnet system length achieved in embodiments of the present invention.

A method of producing a magnet structure according to the presentinvention will now be described. Firstly, an accurate mold is requiredin which to wind the coils. As no former is to be provided, the accuratedimensions and relative spacing of the coils are defined by this mold,which must accordingly be very accurately made, and of a durablematerial, to allow a single mold to be used to produce many magnetstructures. The mold is arranged to be collapsible. Superconducting wireis wound into defined positions in the mold. Typically, these positionswill be recesses in the surface of the mold. A mechanical supportstructure 102, such as a tube of metal such as aluminum or stainlesssteel; or of a composite material such as fiberglass reinforced plastic,is placed over the coils 34 so wound. A layer of fiberglass cloth may beplaced over the outer circumference of the coils before the mechanicalsupport structure 102 is positioned. The layer of fiberglass cloth willprovide thermal diffusion barrier 40. A further mold may be placed overthe coils 34 and the mechanical support structure 102 to form anenclosed mold cavity. The coils and the mechanical support structurewithin the mold are monolithically impregnated with a thermosettingresin. This is allowed to harden and the resin impregnated coils and themechanical support structure, now bonded to the coils, is removed fromthe mold as a single solid piece. The impregnation step is preferablyperformed in a vacuum, to avoid bubbles of air or other gas which mightotherwise be trapped in the coil windings and cause stresses in thefinished piece. The mold may be provided with a lining, such as ofpolytetrafluoroethylene PTFE, to aid in releasing finished articles.

A particular advantage of this method of forming the solenoidal coilarrangement is in that the coils are accurately dimensioned andpositioned relative to each other by the shape of the mold. Themechanical support structure which is bonded onto the coils does notneed to be accurately dimensioned, since the positioning of the coils isdefined by the mold, and the mechanical support structure merely servesto securely retain the coils in their relative positions as defined bythe mold.

This is particularly advantageous since the mold, which must be veryaccurately dimensioned, and is relatively expensive, may be re-usedseveral times to produce a number of similar solenoidal magnetstructures. Hitherto, the former has been the accurately dimensioned,expensive component, and of course can only be used for one magnetstructure. Use of the method of the present invention, using anaccurately machined mold to define the dimensions and relative positionsof the coils, accordingly allows production of accurate solenoidalmagnet structures for a reduced cost, and in reduced time, as comparedto existing methods of production.

According to an aspect of the present invention, a preformed, tubularmechanical support structure 102 is positioned over the coils 34 in amold prior to the impregnation step when thermosetting resin is appliedto monolithically embed the coils and bond them to the mechanicalsupport structure to produce a single solid article. Typically vacuumimpregnation is used. The coils 34 may be over-wound with cloth 40 tobring their outer diameters to the same size as the inner diameter ofthe mechanical support structure, and to provide a thermal diffusionbarrier layer. The cloth will be impregnated with the thermosettingresin during the molding step. Again, the moulds may be provided with alining, such as of polytetrafluoroethylene PTFE, to aid in releasing thefinished articles.

In some alternative embodiments, a wet lay-up process may be employed,where the resin is applied as a coating on the coil conductor, and aspart of the mechanical support structure, either as a coating on themechanical support structure or as an impregnated cloth at layer 40 orsimilar material.

The mechanical support structure 102 may be manufactured as an aluminumextrusion, or formed as rolled and welded tubes of stainless steel; itmay be formed as a filament wound tube of fiberglass reinforced plastic,or may be molded from suitable materials. It may comprise one or morelayers of wire wound into a cylinder and embedded in a thermosetmaterial.

Typically in solenoidal magnet structures, some coils will be ofdifferent internal or external diameter from others. In this case, itmay be necessary to bring all coils to a common diameter to enable themall to be attached to a mechanical support structure of constantinternal diameter. Differences in the relevant diameters of the coilsmay be corrected by use of a filler layer 40 of resin-impregnated clothoverwrap. typically employing glass fiber cloth. This may be added whilethe coil is in a mold, and may be added either as resin impregnatedcloth or as a dry cloth to be impregnated in the mold.

The coils 34 are wound within corresponding parts of the mold, accordingto the method described above. In the mold, a filler material 40 such asresin impregnated glass fiber may be wound over coils in order to fillthe mold to the top. For example, such filler material may be providedto a depth of 5-10 mm. The mold may comprise a collapsible mandrelhaving at least one removable section, allowing the mold to bedisassembled and removed from the interior of the molded coils 20.

The use of relatively inaccurate mechanical support structure 102 forthe coils, being a preformed tube, typically of metal or compositematerial, and molded resin is rendered possible by use of accuratetooling. All of the important relative positions of features of thesolenoidal structure are defined by the mold or other assembly tooling,resulting in a relatively low unit cost of the solenoidal magnetarrangements produced, while the relatively expensive mold and toolingmay be re-used a number of times to produce several solenoidal magnetcoil assemblies.

The final structure may be further strengthened to prevent anysignificant deformation.

FIGS. 4A-4D illustrate several views of steps in a method ofmanufacturing a solenoidal magnet structure according to the presentinvention, in part axial half cross section. In this example, themechanical support structure consists of an aluminum tube 102. FIGS. 4Aand 4B show a partial end-view and a partial axial cross-section,respectively, of a collapsible mold 80, 82, 84 into which coils 34 havebeen wound in predefined positions 88, during the manufacture of asolenoidal magnet coil assembly according to the present invention, withan aluminum tube mechanical support structure 102. The collapsible moldincludes an inner cylinder member 80 which retains tool segments 82, 84.The tube 80 may be a single complete cylindrical tube, or may be dividedinto segments. The tube 80 may be a collapsible mandrel having at leastone removable section. In use, the cylinder 80 and tool segments 82, 84are retained together by detachable mechanical retaining means, such asthe bolts 86 illustrated, to form a generally cylindrical inner surfaceof the mold.

As shown in FIG. 4B, the tool segments 82, 84 have cavities 88 forretaining coils 34 as they are wound onto the mold, and cavities 92 forretaining electrical leads and other service components.

As illustrated in FIG. 4B, over the top of coils 34 and leads andservice components 92, a pre-formed tube 102 of composite material, orof metal, or of other suitable material, is slid over the coils from oneend of the mold. A mold outer 100 is provided to surround the toolsegments and the pre-formed tube 102, and to define mold cavity 99 withthe tool segments 82, 84 and end pieces 98, if any. While the toolsegments 82, 84 must be accurately formed and accurately positioned, itis not necessary to apply such a degree of accuracy to the position andshape of the mold outer 100.

The mold cavity 99 is open in certain locations 108, e.g. at its ends.Openings may also be provided through the mold outer 100. Animpregnation trough 110 is affixed around the mold structure, and athermosetting impregnation resin 122 is forced 124 through the openings108 from the impregnation trough into the mold. The resin 122monolithically impregnates the coils 34, leads and service components92, and any filler material layers 40, 114, and to bond the coils to thepreformed cylindrical mechanical support structure 102 and produce asingle solid article, being the solenoidal magnet structure comprisingcoils and the preformed cylindrical mechanical support structure.

Once the assembly has been fully impregnated and the resin has set, thevarious pieces of the mold 82, 84, 98, 100 are moved away from theresultant single solid article, the molded structure. Firstly, theimpregnation trough 110 and end pieces 98 should be removed from themold. The mold outer 100 may also be removed at this stage, or may beremoved later. Typically, and with reference to FIGS. 4A, 4B, cylinder80 is detached from the tool segments. If the cylinder 80 is in a singlepiece, it can be slid out from the central bore of the assembly. If thecylinder 80 is split into segments, these segments may be dismantled andremoved from the bore of the molded structure. The tool segments 82, 84are then removed from the molded article. In the example shown in FIG.4A, the tool segment 82 is tapered to narrow away from the bore of themold. Such segments should be removed first, to provide clearance forremoval of the remaining tool segments 84.

FIG. 4C shows an example of a solenoidal coil assembly 111 producedaccording to the method described above. The coils 34 are impregnatedwith resin and have dimensions defined by the accurate surfaces of thetool segments 82, 84. They are bonded by the impregnated resin to thepreformed cylindrical mechanical support structure 102. The shearstrength of the mechanical bond between coils and the mechanical supportstructure provided by the resin impregnant is equally effective in bothdirections. The coils are accordingly rigidly held in accurate relativepositions by the mechanical support structure.

As shown in FIG. 4C, holes may be provided through the mechanicalsupport structure, for example to allow access to the coils 34 forelectrical cables, to assist with circulation of coolant, to provide formechanical retention of the structure within a cryostat, and so on. Theholes may be formed by drilling or cutting after the impregnation step,or the mold may be provided with corresponding features to ensure thatthe holes remain open.

FIG. 4D shows a partial cross-section of a shield coil arrangementaccording to an embodiment of the present invention. A shield coil 118is impregnated with resin, and bonded by that resin to a mechanicalsupport structure 120, being a pre-formed cylinder of a compositematerial such as fiberglass-reinforced plastic, or of a metal such asaluminum or stainless steel, or other suitable material. The structureis shown mounted inside a vessel 122, for example a cryogen vessel,provided with locating means 124.

United Kingdom patent application GB2437114 describes methods ofmanufacturing solenoidal magnets, and solenoidal magnets so made, whichbear some resemblance to certain embodiments of the present invention.However, significant differences of the present invention as compared tothis prior art include the following. The preformed cylinders used forthe mechanical support structure in the present invention may beproduced in a “parallel” process. For example, a stock of preformedcylindrical mechanical support structures may be kept, avoiding the riskthat a poorly formed cylindrical mechanical support structure wouldresult in a scrap or reworked magnet, as may be the case with themethods of GB2437114. Any defective cylindrical mechanical supportstructures would be rejected before use, and so could not result in areject solenoidal magnet structure. Each preformed cylindricalmechanical support structure may be tested before use to make sure thatthey meet the requirements of the design before being bonded to magneticcoils. The cylindrical mechanical support structures could be boughtfrom a third party supplier, simplifying the manufacturing process forthe magnet manufacturer. The cylindrical mechanical support structuresthemselves need not be particularly accurately formed, and so may beproduced at relatively low cost.

The use of metal cylindrical mechanical support structures providesfurther advantages, for example that they are recyclable while compositematerials are generally not recyclable; they have a relatively highthermal conductivity, which may allow cooling of the coils through oraround the mechanical support structure; they have a relatively highelectrical conductivity. Such conductive mechanical support structurewill support eddy currents. The eddy currents are dependent on the rateof change of the magnetic field during quenching, ramping and imaging inan imaging system such as a magnetic resonance imaging (MRI) system.Control of the conductivity of the mechanical support structure may beused during the design process to alter stray field bursts during quenchevents; and can be used to control the rate of temperature increaseduring a quench, which would affect the pressure within a cryogen vesselcontaining the magnet. The mechanical support structure should also bestiff and strong. A composite material may be found to provide greaterstrength and/or stiffness per unit volume, or weight, than a metal, whenused for the mechanical support structure of the present invention.

1. A solenoidal magnet structure, comprising: wire wound into coils; a preformed tubular mechanical support structure located over the coils, the whole structure being monolithically impregnated with a thermosetting resin.
 2. A structure according to claim 1, wherein the support structure is preformed by assembly of a plurality of sections, each of arcuate radial cross-section.
 3. A structure according to claim 1, wherein the preformed tubular mechanical support structure carries an axial split.
 4. A structure according to claim 3, wherein the tubular mechanical support structure grips the outer diameter of the coils.
 5. A solenoidal magnet structure, according to claim 1, wherein further electrical leads are retained within the thermosetting resin.
 6. A solenoidal magnet structure, according to claim 1, wherein the wire is a superconducting wire.
 7. A solenoidal magnet structure, according to claim 1, wherein some cons are of different outer diameter from others, and differences in the outer diameters of the coils are compensated by use of a filler layer.
 8. A solenoidal magnet structure, according to claim 7, wherein the filler layer comprises a layer of resin-impregnated cloth.
 9. A solenoidal magnet structure according to claim 1, wherein axially inner coils are of different outer diameter from axially outer coils, and separate mechanical support structures are provided for the axially inner coils and the axially outer coils.
 10. A adenoidal magnet structure according to claim 9, wherein an axially inner mechanical support structure supports axially inner coils, and axially outer mechanical support structures support the axially outer coils.
 11. A solenoidal magnet structure according to claim 9, wherein a first mechanical support structure, of a first internal diameter, supports axially inner coils, and a second mechanical support structure of a second internal diameter greater than the first internal diameter, supports the axially outer coils. 